unserialize() problem

Hello all. I wrote some php pages running on apache1.3/php4.4.0-1 In the login page I make use of a class where I store the users name and some integers (i.e whether the user ia able to see a certain page) and serialize the class to a session var. When you go to a page I unserialize the session var and check if the user is permited to see the page. No when I moved the files to my host server running apache2/php4.3.2 a strange problem appears. When I access the page for the first time the session var is unserialized ok and so I can see the page. If I refresh it this session var (and only this.alla the other session variables wotk ok) is lost and I cannot unserialize it. What may be the problem? Thanks